I. Field of the Invention
The present invention relates to a system for resolving a collision occurring in a local network wherein a data packet is transferred between node stations among a plurality of node stations which use a single transmission line and comprise processors or terminal equipment, in accordance with the contention system, when at least two node stations essentially concurrently transmit the packets.
II. Description of the Prior Art
In the local network using the contention system, (1) the node stations continuously monitor the transmission line and respectively receive data packets when such packets are transmitted thereto, and (2) in response to a transmission request, the corresponding node station transmits data if the transmission line is kept in the nonsignalling state. However, it takes time for any other node station to receive a signal from a given node station after the given station begins transmitting the signal, due to a propagation delay along the transmission line. Therefore, at least two node stations may begin transmitting data before one of them detects a signal transmitted from the other. Thus, a collision takes place on the transmission line.
A system for detecting the collision and for restarting transmission is disclosed in U.S. Pat. No. 4,063,220.
According to this system, an incoming signal is compared with an outgoing signal. If they are the same, the system detects that no collision is occurring. However, if they differ from each other, the system detects that a collision is taking place. In this system, however, if conflicting stations are apart from each other, the signal level of the outgoing signal greatly differs from that of the collision signal, resulting in a failure to detect the collision.
After the collision is detected, retransmission is generally performed. However, in the U.S. patent described above, the conflicting node stations respectively generate independent random numbers, and stop transmission for a predetermined time interval in accordance with the random numbers. When the predetermined time interval has elapsed, retransmission is performed. Thus, the possibility of recollision in the retransmission process is decreased. However, when the range of random numbers is narrowed, the possibility of recollision is increased on the basis of the corresponding processing time. On the other hand, if the range of random numbers is widened, the waiting time must be increased, thus lowering the efficiency of use of the transmission line. Furthermore, the retransmission may not be successfully performcd immediately; a time interval required from the transmission request to the completion of transmission greatly varies in a high traffic time interval.